When the 3.3.0.beta3 version was released, I got this error inconsistently; it failed once, and then the rebuild succeeded. (Not having found this post, I worried about memory corruption! 
) Tonight, doing a rebuild, it consistently failed until I found this post and added DISCOURSE_MAXMIND_ACCOUNT_ID to the existing DISCOURSE_MAXMIND_LICENSE_KEY and then it reliably succeeded.
(I assume something like that maxmind rolled out a change with a blue/green deployment and that earlier I hit different versions of a back end service each time…)